Introduction: Lunar Lava Tube Skylights
Over 200 pits have been identified on the Moon in mare basalt, impact melt deposits, and highland terrain, and interpreted as volcanic lava tube skylights or post-flow features (Wagner and Robinson 2014). Skylights are intriguing as they represent access points to subsurface voids and potentially vast networks of subsurface cavities enjoying substantial isolation and insulation from lunar surface environmental conditions.
Candidate Lava Tube Skylights in Philolaus Crater
We report here on the identification of candidate lava tube skylights along sections of discontinuous sinuous rilles in the impact melt deposits on the floor of Philolaus Crater (Diam ~ 70 km) in the North polar region of the Moon (Figure). The finding is of significance because a) Philolaus Crater is of Copernican age (< ~1.1 Ga old) and the impact melt deposits on its central eastern floor are among the youngest lava flows known on the Moon; and b) Philolaus Crater’s candidate skylights represent potential access points to a large near polar network of subsurface cavities. Relative young age, proximity to the pole, and large subsurface void space mean optimal conditions for potentially accessing cold-trapped subsurface volatiles.
Science and Exploration Philolaus Crater
Exploring Philolaus Crater would represent the first in-situ investigation of a large impact structure formed during the Copernican Period, allowing it to be dated precisely and older lunar crustal remnants excavated by the impact to be examined. Investigating Philolaus’ impact melts would allow assessing modern lunar crustal geochemistry and volatile content. Exploration of Philolaus’ relatively young and little-modified lava tubes might give access to well-preserved volcanic volatiles and cold-trapped subsurface H2O ice. Exploring Philolaus’ skylights and lava tubes would also help prepare for the exploration of analogous features on Mars.
Landing Site Characterization
Relatively smooth landing areas are available on the floor of Philolaus Crater near the candidate lava tube skylights sites. Earth is directly visible from most locations of Philolaus’ impact melt deposits above the potential lava tube network.
Mobility and Sample Return Requirements
Mobility is required to effectively explore and sample the floor of Philolaus Crater. Accessing subsurface cavities through lava tube entrances and skylights would likely require non-wheeled mobility systems such as walkers, hoppers, or rollers (e.g., Thangavelautham et al. 2017). Sample return would be essential to achieving science objectives completely, although an initial in-situ reconnaissance mission without the complexity of sample return would be useful.
References
Thangavelautham, J., M. S. Robinson, A. Taits, T. McKinney, S. Amidan and A. Polak (2017). Flying, hopping pit-bots for cave and lava tube exploration on the Moon and Mars. Proceed. 2nd Int’l Workshop on Instrumentation for Planetary Missions.
Wagner, R. V., and M. S. Robinson (2014). Distribution, formation mechanisms, and significance of lunar pits. Icarus 237, 52-60.
Figure: Philolaus Crater and Canadidate Lava Tube Skylights. A: The Near Side of the Moon; B: The Lunar North Polar Region on the Near Side; C and D: Philolaus Crater (Diameter ~70 km); E: Smooth lava plains on the central eastern floor of Philolaus Crater; F: Network of sinuous rilles on the floor of Philolaus Crater; G: Discontinuous sinous rilles with pitted segments; H and I: Closeups of boxed area in G showing candidate lava tube skylights numbered 1 through 3. (NASA LRO LROC).